During processing of a wafer to form semiconductor devices, accurate temperature control and monitoring of the wafer is critical to an effective process. Variations in the temperature can affect the dimensions of the devices, and therefore, the reproducibility of the process.
To date, the common technique for measuring wafer temperatures is the use of a thermocouple. In using a thermocouple, it is put directly on the silicon wafer or on a monitor chip that sits near the silicon wafer. However, a thermocouple has a finite thermal mass. Therefore, it will modify the temperature of the water. Using the thermocouple on a monitor wafer is also relatively inaccurate, since it is at a distance from the wafer being processed, it cannot truly represent the temperature distribution on the wafer being processed. Further, the wafer being processed is not subject to the disturbing effects of the thermocouple and therefore, the temperature distribution on the wafer being processed and the monitor wafer are not truly identical. Further, use of the thermocouple has a high probability of introducing contamination on the surface of the wafer, because thermocouples are metallic elements.
The more common technique today is used of a pyrometer, a non-contact temperature measurement technique. However, the classic use of pyrometry is extremely sensitive to surface optical properties, and therefore a wafer that has not been processed will have different emission characteristics than a wafer whose surface has been processed. Therefore, the difficulty with this process as presently practiced lies in the changing optical properties of the wafer as the layers are added to the surface of the silicon wafer.